Cell Membrane : Structure and Function
Cell Membrane Structure:
- Early Theory: After chemically analyzing cell
membranes, scientists discovered that the two
main ingredients in the membrane were – Proteins
and Phospholipids.
They then proposed that the Proteins were
sandwiched between two layers of phospholipid
molecules. (as illustrated below)
- Problem: Since the phospholipid tails are
hydrophobic (hate water) this theory did not
explain for how water (hydrophilic) could travel
freely through the membrane.
"Fluid Mosaic Model":
- Accepted Theory: There is a double layer of
phospholipids but the proteins are scattered
through the membrane (mosaic). These
embedded proteins float (raft around) among the
semi-fluid phospholipids (fluid). Many of these
embedded proteins (seen in brown below) span
from one side of the membrane straight through
to the other.
- Water travels through special protein pores
called Aquaporins.
The proteins have polar and non-polar regions
from the amino acid R-Groups, which accounts
for their placement among the phospholipid
bilayer. R-groups that are hydrophobic point
toward the fatty acid chains while R-groups that
are hydrophilic point toward the inner channel
where water flows through.
- Some of the proteins and phospholipids have
carbohydrate chains attached to them.
- Protein + carbohydrate chain = Glycoprotein
- Phospholipid + Carbohydrate chain = Glycolipid
- These carbohydrate chains function as cell
identification markers. Forming somewhat of
a fingerprint on the outside of the cell. A
body will attack cells with the wrong
fingerprint (like a foreign cell, cancer cell, or
infected cell)
Example: Organ rejection after a transplant.
Cell Membrane Function:
- Impermeable – nothing passes through
- Permeable – most things pass through
- Semi-permeable – smaller molecules pass
through but not larger ones. Like a
Strainer/Sieve
- The cell membrane is described as being
SELECTIVELY PERMEABLE
(differentially permeable)-- only certain
desirable molecules can pass through, size is
not the determining factor. The membrane
has mechanisms to allow for the transport of
various types of desirable molecules.
Example : Protein molecules are too large to
fit through the various protein pores, but they
can be transported across the membrane with
vesicle formation.